1. Field of the Invention
This invention relates to a radiation curable optical fiber coating composition which after curing results in a coating having enhanced resistance to moisture and hydrocarbon gel. The invention further relates to a pigmented radiation curable coating composition suitable for use as an outer primary coating for optical fibers.
2. Description of Related Art
Optical fibers are frequently coated with two superposed radiation curable coatings, which together form a primary coating. The coating which contacts the glass is called the inner primary coating and the overlaying coating is called the outer primary coating. In older references, the inner primary coating was called the primary coating and outer primary coating was called the secondary coating, but for reasons of clarity, that terminology was abandoned by the industry in recent years.
The inner primary coating is usually a soft coating providing resistance to microbending. Microbending can lead to attenuation of the signal transmission capability of the coated fiber and is therefore undesirable. The outer primary coating, which is exposed, is typically a harder coating providing desired resistance to handling forces, such as those encountered when the fiber is cabled.
The coating compositions for the inner and outer primary coating generally comprise a polyethylenically unsaturated oligomer in a liquid ethylenically unsaturated medium.
Usually the optical fibers are glass fibers.
Optical glass fibers are weakened upon exposure to water. For example, moisture in air can cause weakening and the eventual breakage of glass fibers. It is therefore desirable that the inner and outer primary coating prevent moisture from attacking the glass substrate. However, many conventional coating compositions have a peak water absorption greater than 1.7% and therefore are not effective in protecting the glass substrate from moisture.
In addition to causing the weakening of glass substrates, moisture can also cause the coating layers to delaminate from each other and/or the glass surface. The delamination of the inner primary coating can result in a weakened glass substrate, because the inner primary coating can no longer protect the glass from attack from moisture.
To avoid moisture damage to the glass surface, it is desirable to provide a coating composition having low water absorption, resistance to delamination from glass, and a low water soak extraction. Moreover a coating composition for optical glass fibers preferably should also provide a cured coating having sufficient adhesion to the glass fiber and yet be strippable for field applications.
For certain applications, conventional coating compositions do not provide cured outer primary coatings having the required combination of sufficient adhesion to the inner primary coating, strippability, resistance to water absorption, and a low water soak extraction.
Furthermore, it is frequently desired to color an outer primary coating to facilitate the selection of the optical fiber which is desired from among many glass fibers in a cable assembly. Published European Patent Application No. 418829 discloses the use of radiation curable ink or a colored solvent borne lacquer to color or overcoat an optical fiber which has already been coated with an inner and outer primary coating. This requires a third coating operation which is undesirable.
It has been proposed to include sufficient pigment for desired coloration directly into the outer primary coating. Such a pigmented outer primary coating is disclosed in published PCT application WO 90/13579, which describes an outer primary coating composition containing pigment particles having a size of less than about 1 micron.
Published Japanese patent application No. 64-22975 describes an ink composition comprising a UV-curable resin and a ethoxylated bisphenol-A-diacrylate. This reference does not disclose using the composition as an outer primary coating nor how to improve the moisture resistance of an outer primary coating. Moreover, the composition disclosed in JP-A-64-22975 is not suitable as an outer primary coating because, when cured, the coating does not have the required toughness to protect the optical glass fiber during handling.
Conventional pigmented outer primary coating compositions, when cured, have insufficient resistance to moisture. When the conventional cured pigmented coating is exposed to water, dimensional changes occur. These dimensional changes can lead to attenuation of the signal transmission capability of the glass optical fiber. Therefore, there is still a need for a coating composition suitable for use as an outer primary coating, which can be pigmented, and which provides a cured coating having low water absorption, a low water soak extraction, and resistance to attack from hydrocarbon gel cable filling material.